Examples of the positive-working photoresist composition include chemically-sensitized resist compositions described in U.S. Pat. No. 4,491,628, and EP 29,139. The chemically-sensitized positive-working resist composition is a pattern-forming material which, when irradiated with radiation such as far ultraviolet rays, allows the exposed area to generate an acid in the presence of which it then undergoes reaction to change the solubility of the area irradiated with actinic radiation and the other area in a developer so that a pattern is formed on a substrate.
Examples of these pattern-forming materials include a combination of a compound which undergoes photodecomposition to generate an acid with acetal or O, N-acetal compound (JP-A-48-89003 (The term "JP-A" as used herein means an "unexamined published Japanese patent application")), a combination of a compound which undergoes photodecomposition to generate an acid with orthoester or amide acetal compound (JP-A-51-120714), a combination of a compound which undergoes photodecomposition to generate an acid with a polymer having acetal or ketal in its main chain (JP-A-53-133429), a combination of a compound which undergoes photodecomposition to generate an acid with an enol ether compound (JP-A-55-12995), a combination of a compound which undergoes photodecomposition to generate an acid with an N-acyliminocarbonate compound (JP-A-55-126236), a combination of a compound which undergoes photodecomposition to generate an acid with a polymer having an orthoester group in its main chain (JP-A-56-17345), a combination of a compound which undergoes photodecomposition to generate an acid with a tertiary alkyl ester compound (JP-A-60-3625), a combination of a compound which undergoes photodecomposition to generate an acid with a silyl ester compound (JP-A-60-10247), and a combination of a compound which undergoes photodecomposition to generate an acid with a silyl ether compound (JP-A-60-37549, JP-A-60-121446). These compounds each exhibit a quantum yield of more than 1 in principle and thus show a high photosensitivity.
As a system which normally stays stable with time but undergoes decomposition to become alkali-soluble when heated in the presence of an acid there may be used a combination of a compound which generates an acid when exposed to light described in JP-A-59-45439, JP-A-60-3625, JP-A-62-229242, JP-A-63-27829, JP-A-63-36240, JP-A-63-250642, Poly. Eng. Sci., vol. 23, p. 12 (1983), ACS. Sym. vol. 242, p. 11 (1984), Semiconductor World Nov. 1987, p. 91, Macromolecules, vol. 21. p. 1475 (1988), SPIE, vol. 920, p. 42 (1988), etc. with a tertiary or secondary carbon (e.g., t-butyl, 2-cyclohexenyl) ester or carbonic ester compound. Such a system, too, has a high sensitivity and shows little absorption in far ultraviolet range. Thus, this system can be a system suitable for the reduction of wavelength of light from light source enabling ultrafine working.
JP-A-2-19847 discloses a photoresist composition containing a resin having phenolic hydroxyl groups in poly (p-hydroxystyrene) entirely or partially protected by tetrahydropyranyl group.
Similarly, JP-A-4-219757 discloses a photoresist composition containing a resin having phenolic hydroxyl groups in poly(p-hydroxystyrene) substituted by acetal group in a proportion of from 20 to 70%.
Further, JP-A-5-249682 discloses a similar photoresist composition comprising an acetal-protected resin.
Moreover, JP-A-8-123032 discloses a photoresist composition comprising a ternary copolymer containing groups protected by acetal group.
Further, JP-A-5-113667, JP-A-6-266112, JP-A-6-289608, and JP-A-7-209868 disclose a photoresist composition comprising hydroxystyrene and (meth)acrylate copolymer.
However, these resist compositions exhibit a drastic dimensional change in resist pattern with time between exposure and post bake (post exposure delay time; hereinafter referred to as "PED") and leave something to be desired in resolution.
Further, JP-A-8-253534 discloses a photoresist composition comprising a partially crosslinked polymer containing group substituted by acetal group. JP-A-8-305025 discloses a radiation-sensitive composition comprising a polymer containing a specific acid-cleavable acetal group in polymer main chain or as a crosslinking member between two or more polymer chains. JP-A-10-31310 discloses a radiation-sensitive composition comprising a copolymer having specific repeating units (1) and (2) and a branched chain unit.
However, these photoresist compositions comprise a hydroxystyrene skeleton and thus are disadvantageous in that they exhibit a deteriorated adhesivity to substrate and the resulting lone pattern can be peeled off the substrate during development (hereinafter referred to as "loss of lone pattern"). Thus, these photoresist compositions leave something to be desired.